1. Field of the Invention
The present invention relates generally to foams having a solid phase matrix with air-filled pores. More specifically, the present invention relates to starch-based microcellular foams having high pore volume, low density, and high surface area. The invention further relates to methods of preparing the foams from starch aquagels. The starch-based microcellular foams are particularly useful for encapsulation and release of compounds such as flavoring compounds, fragrances, and pharmaceuticals.
2. Description of the Art
Starch is an important agricultural product that, in the United States, is derived primarily from corn, potatoes, and wheat. In its raw state, starch is in the form of dense granules that range in size from 2 microns in wheat starch to over 100 microns in potato starch. Granular starch is utilized in many food and non-food products and is often chemically modified to further expand its uses. Starch may be heated in the presence of water to cause the starch granules to gelatinize. Gelatinization is the swelling and hydration of starch granules which result in the disruption of molecular order within the granule and solubilization of the starch molecules.
Starch granules are generally primarily composed of two polymers: amylose (linear) or amylopectin (branched), with the largest portion being amylopectin (70-85%). However, starch granules from certain plant varieties are almost entirely amylopectin (98%) while others may contain 45% to 80% amylose. Amylose is a polymer of (1-4)-linked .alpha.-D-glucopyranosyl units with a molecular weight that varies depending on the source of the starch but is generally much smaller than that of amylopectin. Amylopectin has short branches on only about 4% of the D-glucosyl residues. Amylopectin molecules range from about 50 to 100 million daltons in molecular weight. Aqueous starch solutions that have been heated to initiate gelatinization can be cooled to initiate gelation, the process of gel solidification that occurs as the aqueous starch gels are cooled or aged. Gelation appears to be dominated by the amylose content of starch even though it may not be the major fraction. Gelation takes place as molecular reassociation of the hydrated and dispersed starch molecules occurs, presumably through hydrogen bonding in the aqueous gel.
The gelation properties of starch have been exploited in the food industry to make products with desirable texture. High amylopectin starches have been used to make puddings with soft, creamy textures. Wheat and regular dent corn starches contain 23 to 28% amylose and can be gelatinized and allowed to undergo the gelation process to form firm aqueous gels. High amylose corn starches are also commercially available which form unusually firm aqueous gels at low concentrations.